1.Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device with high breakdown voltage having an excellent radiation resistance.
2. Description of the Prior Art
In a conventional MOS transistor, ionization is induced in a silicon oxide film if it is irradiated with beams of radiation such as represented by .gamma. rays. Of both positive and negative electric charges that are created by the ionization, the negative charges (electrons) that have a large mobility are annihilated immediately, whereas the positive charges that have a small mobility are left behind within the oxide film. These positive charges are gradually trapped in the area between the oxide film and a substrate, forming fixed charges. An inversion takes place below the oxide film due to these charges, and results in the formation of a parasitic MOS transistor. In particular, this phenomenon is conspicuous in a thick (about 7 to 10 .mu.m) silicon oxide film such as a field oxide film, causes an undesired leakage current to flow between the source and the drain or between circuit elements, generating a deterioration in the characteristics of the transistor.
In a conventional integrated circuit using MOS transistors, in order to enhance the radiation resistance between the MOS transistors, a high concentration region having polarity different from that of the source-drain region is formed below a thick oxide film as an isolation region between the MOS transistors as shown, for example, in U.S. Pat. No. 4,748,489, to prevent the flow of a leakage current between the circuit elements. However, even if the leakage current between the transistors is prevented, there still remains a drawback in that it is not possible to fully prevent the leakage current between the source and the drain in the individual transistor.